Formula
Metric BMI = weight kg ÷ height m². US BMI = 703 × weight lb ÷ height in². Healthy-weight range = BMI boundary × height m². BMI Prime = BMI ÷ 25. Ponderal Index = weight kg ÷ height m³.
CalculationTime
To calculate time is to make the future less invisible.
Worked example
For 154 lb and 5 ft 9 in: total height is 69 in. BMI = 703 × 154 ÷ 69² = 22.74 kg/m². At 69 in, the adult healthy-weight range using BMI 18.5-24.9 is about 125.3-168.6 lb.
Professional note
Master’s Tip: keep BMI, category, height, weight, age context, healthy-weight range and the “not a diagnosis” limitation together. BMI is useful as a quick screening index, but waist measurement, body composition, pregnancy status, ethnicity, medical history and clinician judgement can change the interpretation.
Regional and unit assumptions
Standard or basis: this page uses the common adult BMI formula and widely used adult screening bands: under 18.5, 18.5-24.9, 25.0-29.9 and 30.0 or above. It also shows BMI Prime against 25 and Ponderal Index as a height-adjusted cross-check. It does not provide diagnosis, treatment or personalised health advice.
Assumptions and limitations
- The calculator supports metric BMI arithmetic and the equivalent US customary formula using the 703 conversion factor.
- Height must be greater than zero because the formula divides by height squared.
- Adult BMI categories are screening categories for adults aged 20 and over, not a diagnosis or individual health assessment.
- The healthy-weight range uses common adult BMI boundaries of 18.5 to 24.9 and is rounded for practical display.
- BMI can be misleading for children, pregnancy, athletes, older adults and people with unusual body composition; use clinical guidance for health decisions.
Trust boundary
What this calculator does NOT do
- Diagnosing weight status, disease risk or treatment needs
- Replacing clinician advice, growth charts, pregnancy care or body-composition assessment
- Setting a personalised target weight without medical context
When to stop using this calculator
- Use child and teen BMI percentiles, not adult bands, for anyone under 20.
- Ask a qualified professional before using BMI to make health decisions during pregnancy, serious illness, eating-disorder risk, athletic training or major weight change.
- Consider waist measurement, ethnicity, medication, fitness, muscle mass and medical history before interpreting the number.
Methodology & Accuracy
How this calculator is checked
CalculationTime pages are built around visible arithmetic: the formula, assumptions, worked example and practical limitations are shown so the result can be checked rather than simply trusted.
Formula used
Metric BMI = weight kg ÷ height m². US BMI = 703 × weight lb ÷ height in². Healthy-weight range = BMI boundary × height m². BMI Prime = BMI ÷ 25. Ponderal Index = weight kg ÷ height m³.
Standard or basis
Standard or basis: this page uses the common adult BMI formula and widely used adult screening bands: under 18.5, 18.5-24.9, 25.0-29.9 and 30.0 or above. It also shows BMI Prime against 25 and Ponderal Index as a height-adjusted cross-check. It does not provide diagnosis, treatment or personalised health advice.
Where a calculator follows a named legal, trade or industry standard, that standard is cited visibly. Otherwise the page uses transparent general arithmetic and states its limits.Master's Tip
Master’s Tip: keep BMI, category, height, weight, age context, healthy-weight range and the “not a diagnosis” limitation together. BMI is useful as a quick screening index, but waist measurement, body composition, pregnancy status, ethnicity, medical history and clinician judgement can change the interpretation.
Related calculators
Questions
How do you calculate BMI?
In metric units, divide weight in kilograms by height in metres squared. In US units, multiply pounds by 703 and divide by height in inches squared.
What is a healthy BMI range for adults?
Common adult screening bands place 18.5 to 24.9 in the healthy-weight range, but individual health needs clinical context.
What healthy weight range does this calculator show?
It multiplies height squared by BMI 18.5 and 24.9 to show the approximate adult weight range that maps to the common healthy-weight BMI band.
What is BMI Prime?
BMI Prime is BMI divided by 25. A BMI Prime of 1.00 sits at the common adult overweight threshold.
What is Ponderal Index?
Ponderal Index divides weight in kilograms by height in metres cubed. It is a secondary body-size index sometimes used as a cross-check because BMI can scale imperfectly at very short or very tall heights.
Is BMI a diagnosis?
No. BMI is a simple screening measure. It does not directly measure body fat, fitness, muscle mass, pregnancy status or individual medical risk.
Can children use this BMI calculator?
The arithmetic can be calculated, but children and teenagers need age-and-sex BMI percentile charts rather than adult BMI category bands.
Calculation note
BMI is a compact height-and-weight index. It is useful because the formula is simple and repeatable, but a competitive calculator must also show units, category, healthy range and limitations so the number is not mistaken for a diagnosis.
BMI is an index, not a direct body-fat measurement
BMI compares weight with height squared. That makes it easy to calculate across large groups, but it does not directly measure body fat, muscle, bone density or individual health risk.
Adult category bands are screening bands
Public-health pages commonly present adult BMI bands such as underweight, healthy weight, overweight and obesity. Those bands can help with screening and education, but personal interpretation should consider age, sex, ethnicity, medical history, waist measurement, body composition and professional advice.
Healthy weight range is a formula range, not a prescription
The healthy-weight range on this page reverses the BMI formula using the common adult 18.5-24.9 band. It is useful for education and context, but it is not a personalised target weight or medical instruction.
BMI Prime and Ponderal Index add context
BMI Prime compares the result with the adult overweight threshold of 25. Ponderal Index divides weight by height cubed and gives a second body-size cross-check. Both help explain the number, but neither replaces clinical assessment.
Sources and further readingCDC: About Adult BMIWHO: Body mass index - BMI